It is known to use an evacuated space between glass panels to provide an insulated window that is superior over standard double glazing without evacuation of the space between the glass panels. Examples of such evacuated glass panels or envelopes are shown in Johnson, U.S. Pat. No. 4,185,616 dated 29 Jan., 1980, Kenny, U.S. Pat. No. 4,184,480 dated 22 Jan., 1980 and Benson, U.S. Pat. No. 4,683,154 dated 28 Jul., 1987.
In any such design, it is necessary to separate the glass sheets or panes forming the evacuated glass panels. If flat panes are used, there is a tendency for the panes to bend under atmospheric pressure and break or touch, destroying the insulating utility of the envelope. To solve this problem, Benson discloses the use of glass beads located between the panes. Kenny's solution is to provide a sinusoidally corrugated glass envelope, which, in order to obtain maximum uniform tensile strength, has upwardly extending corrugations each contoured laterally of the window to form a paraboloid. Johnson also discloses the use of a curved panel, and, to eliminate bending stresses in the panel, created by the evacuation of the interior of the panel, the perimeter of the panel is tensioned. Thus, in Johnson, the transparent panel is held in tension by using a complexly configured glass panel, and a complexly shaped basin to receive the glass panel, and to exert pressure on the interior complexly shaped panel.
Such solutions to the problem of providing an evacuated glass envelope as are shown in Johnson and Kenny require complexly shaped glass panels, or, as shown in Benson, }requires glass beads or spacers. Such configurations result in the thermal panels being difficult to construct.